1. Field of the Invention
The present invention relates to laser systems based on crystals with the chemical formula: RE-doped MA.sub.2 X.sub.4, where M includes divalent ions such as Mg, Ca, Sr, Ba, Pb, Eu, or Yb; A is selected from trivalent ions including Al, Ga, and In; X can be one of the chalcogenide ions S, Se, and Te; and RE stands for the trivalent rare earth ions. The invention relates particularly to the use of the aforementioned crystals as the gain element in a laser or optical amplifier. The invention relates more particularly to the operation of this laser at a laser wavelength approximately equal to or longer than 3 microns.
2. Description of Related Art
Solid state lasers based on rare earth laser ions that directly provide mid-infrared output at &gt;3 micron wavelengths have previously been developed, although none of these materials are in common use because they do not have compelling performance characteristics. U.S. Pat. No. 5,746,946 by Bowman et al. dated May 5, 1998, disclosed the utility of Er.sup.3+ as a laser ion that operates from 3-5 microns. Bowman et al. also mention the importance of using a low-phonon-frequency host material in order to obtain efficient mid-infrared emission. U.S. Pat. No. 5,535,232 (Bowman et al., dated Jul. 9, 1996) discloses another mid-infrared laser material, involving the use of Pr.sup.3+ rare earth ions yielding laser output at 5 and at 7 microns. The present invention calls for the use of sulfide, selenide or telluride crystalline hosts doped with rare earth ions to generate infrared laser light beyond 3 microns. Laser light is commonly generated at less than 3 microns using the oxide and fluoride based crystals that have been employed for many decades, [for example see Caird and Payne, "Crystalline paramagnetic ion lasers," in Handbook of Laser Science and Technology (CRC Press, Boca Raton, 1991), pp. 1-100]. It is worthwhile to mention that a rare earth ion has never previously been reported to lase in a sulfide, selenide, or telluride crystalline host medium.
Rare earth ions have previously been incorporated into various glassy materials (as opposed to crystalline) with the intent of producing laser action. Dy.sup.3+ -doped glassy waveguide lasers have been disclosed in European Patent No. EP 0 756 767 B1 (Samson et al., filed on Apr. 24, 1995) and lasers comprised of Ge--Ga--S glasses doped with rare earth ions are disclosed in U.S. Pat. No. 5,379,149 (Snitzer et al., dated Jan. 3, 1995). Both of these inventions specifically relate to the use of waveguides and glasses. In contrast, the present invention is a specific crystalline composition of matter employed as the gain medium in a laser system.
As noted above, the subject materials of this patent application have the general formula RE-doped MA.sub.2 X.sub.4 (where M includes divalent ions such as Mg, Ca, Sr, Ba, Pb, Eu, or Yb; A is selected from trivalent ions including Al, Ga, and In; X can be one of the chalcogenide ions S, Se, and Te; and RE stands for the Ce.sup.3+, Nd.sup.3+, Pm.sup.3+, Sm.sup.3+, Eu.sup.3+, Gd.sup.3+, Tb.sup.3+, Dy.sup.3+, Ho.sup.3+, Tm.sup.3+ and Yb.sup.3+ trivalent rare earth ions). A number of published papers in the scientific literature contain discussions of the visible luminescence of RE-doped thiogallates, in particular for CaGa.sub.2 S.sub.4 and SrGa.sub.2 S.sub.4 --for example see Garcia et al., "Photo- and cathodoluminescent properties of erbium-doped thiogallates," J. Electrochem. Soc.: Solid-State Science and Technology 129, 2063-2069 (1982); Peters, et al., "Luminescence and structural properties of thiogallate phosphors Ce.sup.3+ and Eu.sup.2+ -activated phosphors," J. Electrochem. Soc.: Solid-State Science and Technology 119, 230-236 (1972); Garcia, et al., "Charge transfer excitation of the Nd.sup.3+, Sm.sup.3+, Dy.sup.3+, Ho.sup.3+, Er.sup.3+, and Tm.sup.3+ emission in CaGa.sub.2 S.sub.4," J. Luminescence 33, 15-27 (1985); Georgobiani, et al., "Photoluminescence of rare earths in CaGa.sub.2 S.sub.4," Inorganic Materials 31, 16-19 (1995). There are also several patents which disclose the use of these RE-doped thiogallate hosts as candidates for electroluminescent devices: U.S. Pat. No. 5,612,591 (Katayama et al., Mar. 18, 1997) and U.S. Pat. No. 5,747,929 (Kato, et al., May 5, 1998). Of the many papers and patents appearing in the scientific literature, no previous researchers have discussed the option of utilizing the RE-doped MA.sub.2 X.sub.4 crystals as gain media to generate mid-infrared light. While one versed in the art could be expected to know that RE ions in general serve as laser generators, the special ability of these materials to operate in the mid-infrared range is only evident when their phonon spectra are considered, as discussed below. The fact that these crystals' phonon spectra contain no high-frequency vibrations is crucial, and qualifies them as "low-phonon-frequency hosts."
An important principle of solid-state physics that had been worked out several decades ago involves the impact of the phonon spectrum on the luminescence efficiency, [for example see Weber, "Multiphonon relaxation of rare-earth ions in yttrium orthoaluminate," Phys. Rev. B 8, 54 (1973)]. The basic theory indicates that, in order for materials to emit at long wavelengths, the highest phonon frequencies of the host medium must be less than about 0.20-0.25 times the light frequency. The sulfides, selenides, and tellurides that discussed above have suitably low phonon frequencies to allow for efficient emission beyond 3 microns, while the standard oxide and fluoride crystals are not as suitable for this purpose. For example, the frequency (in wavenumber units) of 5 micron light is 2000 cm.sup.-1, and 20% of that corresponds to 400 cm.sup.-1, the approximate maximum phonon vibrational frequency a host material should possess in order to facilitate efficient 5 micron luminescence.
The present invention combines the knowledge that rare earth ions can lase in the mid-infrared with a preferred host medium (having high optical quality and low phonon frequency).